229.1.0: Import FoundationEssentials in the file session store

The file store was the last code importing umbrella Foundation; every
other source and test already takes FoundationEssentials when it exists.

Three APIs the split module does not carry needed replacing. String(format:)
gives way to nibble-wise String(_:radix:), which emits the same bytes for
every input, so hashed session filenames are unchanged and existing stores
still resolve. The tests list a directory by path rather than by URL, and
read posix permissions through a helper that accepts either the UInt of
FoundationEssentials or the bridged NSNumber of umbrella Foundation.